Pardon my ignorance, I don't have a pump, yet. But wouldn't the input on the high side mean liquid would flow down via gravity right into the head, thus priming it?

The basic challenge to priming the pump is that the air in the pump must be displaced by the liquid. If gravity cannot overcome the air pressure, the pump will not prime. So if the inflow is directly above, and the outflow is directly below, it can be difficult for the liquid to overcome the air because the air is an opposing force. Having the inflow lower than the outflow better enables the air to escape, so the liquid can fill the pump. Once the liquid fills the pump, the pump itself should be able to overcome the remaining air.

That's the gist of it. Some people use bleeder valves to make it even easier for the air to escape, but the basic configuration of outflow higher than inflow is the recommended approach, regardless.

The basic challenge to priming the pump is that the air in the pump must be displaced by the liquid. If gravity cannot overcome the air pressure, the pump will not prime. So if the inflow is directly above, and the outflow is directly below, it can be difficult for the liquid to overcome the air because the air is an opposing force. Having the inflow lower than the outflow better enables the air to escape, so the liquid can fill the pump. Once the liquid fills the pump, the pump itself should be able to overcome the remaining air.

That's the gist of it. Some people use bleeder valves to make it even easier for the air to escape, but the basic configuration of outflow higher than inflow is the recommended approach, regardless.

Excellent info, thanks. I have a pump on order right now and just assumed I'd put the input above output. I may mount it "upside down" now based on this info.

Regarding the pump setup - the issue I've seen is not the initial prime - as the liquid flows out of your kettle and through the pump and then back out the discharge of the pump, it will clear most, if not all the air. The issue is air pockets - any bubbles that get sucked into the pump, or any cavitation, anything that causes air to enter the pump head will not clear. The bubbles will try to go "up", while the liquid is flowing "down", and basically you'll have stuck bubbles. If you have the discharge as the high point, any and all remaining bubbles will want to go in the same direction as the flowing liquid and the pump will run better and happier. Because pumps have feelings, too.

Regarding the pump setup - the issue I've seen is not the initial prime - as the liquid flows out of your kettle and through the pump and then back out the discharge of the pump, it will clear most, if not all the air. The issue is air pockets - any bubbles that get sucked into the pump, or any cavitation, anything that causes air to enter the pump head will not clear. The bubbles will try to go "up", while the liquid is flowing "down", and basically you'll have stuck bubbles. If you have the discharge as the high point, any and all remaining bubbles will want to go in the same direction as the flowing liquid and the pump will run better and happier. Because pumps have feelings, too.

Good point, although I have seen failure to prime also.

Off-topic, but when I rack to keg after a 33F cold crash, at about 65F ambient temperature, I encounter a similar physical phenomenon. The CO2 comes out of solution and rises, opposing the gravitational force acting on the liquid, resulting in slower racking.

What do you do for separating hops and hot break from the wert? I see you have a setup to allow for whirlpooling but is that not opposite if what you want with a center drain?

Pretty sure his center drain is for CIP (clean in place). That allows you to rinse, scrub, etc. and everything just drains out the bottom, like your bathtub, without having to tip the vessel over, etc.